1. Field of the Invention
The present invention relates to an optical scanner for scanning laser beams, a scanning lens, which is an optical component for scanning laser beams, and an image forming apparatus for forming an image by scanning laser beams, and more particularly to an optical scanner having an one-piece comprised scanning lens having the function of an F.theta. lens used in a laser scanning optical system, a scanning lens and an image forming apparatus.
2. Description of the Related Art
A laser beam scanner is used for a laser printer or a digital copying machine. This laser beam scanner generally comprises: as shown in, for example, FIG. 3, a laser beam source 11; a collimator lens 13 for converting divergent light from the laser beam source 11 into substantially parallel light beams; a slit 15; a cylinder lens 17 for focusing only parallel light beams in the slow scan direction; a light deflector 19 having a reflective surface near a focal position focused by the cylinder lens 17 for rotating at a constant angular velocity; an F.theta. lens 21 for moving a light beam deflected at a constant angular velocity on a surface to be scanned at a constant velocity and focusing the light beam in the fast scan direction; and an inclination-correcting cylinder mirror 23 for causing the reflective surface of the light deflector 19 to be conjugate with the surface to be scanned in the slow scan direction and focusing the light beam on the surface to be scanned (see Japanese Published Unexamined Patent Application No. 6-18803).
In the above-described laser beam scanner, the following three functions are requested for the optical system (scanning optical system) on or after the light deflector. The first function is to convert a light beam deflected by the light deflector at a constant angular velocity so as for the light beam to scan on the surface to be scanned at a constant velocity. The second function is to cause the reflective surface of the light deflector to be conjugate with the surface to be scanned in order to prevent the scanning line position on the surface to be scanned from fluctuating in the slow scan direction by means of an error when a plurality of reflective surfaces, which the light deflector has, incline in the slow scan direction. The third function is to cause both substantially parallel light beams in the fast scan direction and divergent light beams in the slow scan direction to form an image on the surface to be scanned.
Such functions required for the scanning optical system have generally been implemented by a plurality of optical elements (two pieces of F.theta. lenses and a cylinder mirror in the above-described example). In order to reduce the component count in the laser beam scanner and to make the apparatus size compact, however, there have been proposed a large number of techniques for providing one piece of lens with the above three functions (Japanese Published Unexamined Patent Application Nos. 62-138823, 4-50908, 8-190062 and the like) . Almost all of these techniques are applied to an optical system in which an incident light beam on a light deflector and a reflective light beam thereby are in a fast scan surface including the normal of a deflecting reflective surface.
As the scanning optical system using an F.theta. lens, there are an underfilled optical system and an overfilled optical system.
In the underfilled optical system, the width of a light beam becoming incident on a polygonal rotating mirror is smaller than the polygonal rotating mirror, and the light beam does not protrude outwardly of the reflective surface while the surface to be scanned is being scanned by the light deflector (see the Japanese Published Unexamined Patent Application No. 6-18803). In the underfilled optical system, as shown in FIG. 4, all light beams becoming incident on the light deflector are used for deflection and scanning, and therefore, the light beam energy is effectively used. Since the width of a light beam becoming incident on the F.theta. lens irrespective of any scanning angle is constant, there is a merit that the diameter and quantity of light of an optical spot to be imaged on the surface to be scanned are uniform. Since, however, the width of the reflective surface in the light deflector must be made sufficiently larger than that of the incident light beam, there is a demerit that the diameter of the polygonal rotating mirror will be dramatically large if a number of the reflective surfaces is increased to speed up or the width of the incident light beam on the light deflector is made larger to reduce the optical spot diameter on the surface to be scanned.
On the other hand, in the overfilled optical system, the width of a light beam becoming incident on a polygonal rotating mirror is larger than the polygonal rotating mirror, and the light beam always protrudes outwardly of the opposite sides of the reflective surface used for scanning while the surface to be scanned is being scanned by the light deflector (see the Japanese Published Unexamined Patent Application No. 8-171070). In the overfilled optical system, as shown in FIG. 5, part of a light beam becoming incident on the light deflector is used for deflection and scanning, and therefore, a loss in the light beam energy is significant. Since the width of a light beam becoming incident on the F.theta. lens changes depending upon the scanning angle, there is a demerit that the diameter of an optical spot imaged on the surface to be scanned and power significantly change. Since, however, the width of the reflective surface in the light deflector becomes an opening width of the light beam becoming incident on the F.theta. lens and the width of the reflective surface of the polygonal rotating mirror becomes smaller, there is a merit that the diameter of the polygonal rotating mirror required is small even if an attempt is made to increase the number of reflective surfaces or to make the optical spot diameter on the surface to be scanned smaller.
In order to reduce the respective demerits of the above-described underfilled and overfilled optical systems, there has been proposed a method of causing a light beam to become incident on the light deflector from near the central portion of the deflection angle of the light deflector as also shown in FIG. 6 (Japanese Published Unexamined Patent Application Nos. 63-204221 and 9-230274). In the case of the underfilled optical system, there is a merit that the width of the reflective surface can be made smaller and the diameter of the polygonal rotating mirror becomes smaller because the light beam width projected on the reflective surface becomes small. Also, in the case of the overfilled optical system, there is a merit that a change in the width of the incident light beam on the F.theta. lens based on deflecting reflection becomes smaller and heterogeneity in the optical spot diameter and the quantity of light on the surface to be scanned is improved. Since it is actually necessary to separate the incident light beam on the light deflector from the deflected and reflected light beam, the light beam is caused to obliquely become incident in the slow scan direction although on the optical axis of the F.theta. lens in the fast scan direction.
Also, for the optical scanner, there has been proposed a technique using a plurality of light beams in order to correspond to plural colors (see the Japanese Published Unexamined Patent Application No. 10-73778). According to this technique, as shown in FIG. 7, a plurality of light beams arranged in the slow scan direction are incident on a single light deflector and a plurality of light beams reflected by the light deflector are guided to different positions on the surface to be scanned by respective different mirrors not to thereby increase the number of the light deflectors. Also, in order to cause the plurality of light beams arranged in the slow scan direction to become incident on the same reflecting point on the light deflector, the respective light beams are caused to become obliquely incident (hereinafter, referred to as oblique-incidence) on the light deflector.
[Problems to be solved by the Invention]
To cause a light beam to become oblique-incident on the light deflector in the slow scan direction has a merit, but when oblique-incident on the light deflector as shown in FIG. 8, the angle of a light beam deflected and scanned by the light deflector in the slow scan direction is changed by the deflection angle, and therefore, the lens incident position and angle of the deflected light beam becoming incident on the scanning optical system in the slow scan direction are changed by the deflection angle of the light deflector (the Japanese Published Unexamined Patent Application No. 10-73778). For this reason, in a case where the scanning optical system is constructed of one piece of lens, the curved scanning line on the surface to be scanned and the deteriorated image-formation performance of the optical spot could not be avoided because the locus of the light beam is curved with respect to the center line of the F.theta. lens, which is a straight line in the lens shape conventionally proposed. In order to resolve such problems, as shown in FIGS. 9 and 10, the center line of the F.theta. lens is incurvated to meet the locus of the incident light beam on the F.theta. lens curved by the oblique-incidence to thereby prevent the deteriorated image-formation performance, and further at least one of the lens surfaces is shifted in the slow scan direction to reduce the curved scanning line on the surface to be scanned.
In a case, however, where the shape of the lens is incurvated to meet the curved locus of the lens incident light beam, the F.theta. lens is to satisfy the performance only with respect to the light beam which becomes incident on the F.theta. lens at a predetermined angle and position. Therefore, the same number of lenses as the number of light beams are required with respect to a plurality of light beams having different oblique incident angles, and it was not possible to reduce the number of the lenses by sharing the lens with respect to a plurality of light beams. Also, since a plurality of lenses are arranged in the slow scan direction, it is necessary to make the oblique incident angle of each light beam larger in order to cause a light beam to become incident on each lens, and as a result, the center line of the lens must be curved by a large amount, leading to a problem that it is difficult to manufacture the lens.
Also, in order to reduce the angle of oblique incidence, there is also a technique of causing both a light beam becoming incident on the light deflector and a light beam deflected and reflected by the light deflector to pass through a scanning lens (see Japanese Published Unexamined Patent Application Nos. 63-204221 and 9-96773). In a case where the scanning lens is constructed of one piece, optical paths of both the incident light beam on the light deflector and the light beam deflected and reflected approach each other from the scanning lens toward the surface to be scanned in the slow scan direction because the scanning lens is a convex lens, and it is difficult to separate the incident light beam from the deflected light beam. For the reason, the angle of oblique incidence had to be still made larger.